1. Field of the Invention
The present invention relates to coherent optical communication techniques and, more particularly, to a polarization switching light source, an optical receiver, and a coherent optical transmission system.
2. Description of the Related Art
Recently, rapid progresses have been made in coherent optical communication techniques in which heterodyne detection or homodyne detection is performed by utilizing the properties of light as a wave (e.g., T. Okoshi and K. Kikuchi, "Coherent Optical Fiber Communications", KTK Scientific Publishers, Tokyo, 1988). In coherent optical communication, a local oscillation source is prepared on the reception side to receive a beat signal resulting from interference between signal light and local oscillation light and appearing in a photodetector. Since the spectrum purity of signal light or local oscillation light is higher than that in a conventional direct detection system, optical frequency division multiplexing (optical FDM) with high density can be realized. The realization of an optical FDM, in combination with the recent remarkable advances in the optical amplifier, enables long-range nonrepeated (or multiple branch) optical communication of a capacity much larger than that of a conventional system. Such a technique, therefore, is expected to be applied to various types of communication systems, e.g., a broad band ISDN, a high definition optical CATV, and a metropolitan area network (MAN).
Several problems, however, are posed in the practical application of coherent optical communication. The first problem is associated with polarization matching; the second problem, phase noise; the third problem, the band width of a receiver; and the fourth problem, image components which interfere with the realization of a high density optical FDM. These problems will be sequentially described below.
(a) Problems associated with polarization matching
In general, signal light transmitted through an optical fiber varies in polarization state depending on the temperature of the fiber, a stress acting thereon, and other disturbances. Assuming that local oscillation light is in a constant polarization state, a variation in polarization state of signal light corresponds to the intensity variation of a beat signal resulting from interference. In an extreme case, if the polarization states of signal light and local oscillation light are orthogonal, the resulting beat output becomes zero. In order to solve this problem, the following means are proposed:
(1) the use of a polarization maintaining fiber PA1 (2) mechanical or electrooptical control of polarization states PA1 (3) polarization diversity reception PA1 (4) polarization scrambling or polarization switching PA1 (a) a polarization bistable semiconductor laser PA1 (b) a polarization switch using an external modulator (in the method proposed by T. G. Hodgkinson et al.) PA1 (c) a polarization switch using a laser frequency switch (in the method proposed by I. M. I. Habbab et al.)
However, with regard to (1) the use of a polarization maintaining fiber, problems are posed in terms of a connection method, cost, and the like. In addition, with regard to (2) control of polarization states in a fiber, problems are posed in terms of a trade-off between infinite tracking properties and complexity of a controller, reliability, size, difficulty in a multichannel optical FDM, difficulty in cold start, and the like.
For the above-described reasons, (3) polarization diversity reception is widely used (e.g., L. G. Kazovsky, "Phase- and Polarization-Diversity Coherent Optical Techniques", Journal of Lightwave Technology, Vol. 7. No. 2 pp. 279-292. February 1989). In this method, two independent polarization components are separately received (multi-port reception), and the received components are electrically synthesized. The method, however, requires a complicated optical system for a receiver, and also requires two light receivers and two intermediate frequency (IF) circuits. In consideration of the application of the method to a subscriber's system such as an optical CATV system, aside from a trunk transmission system, problems are posed in terms of adjustment of an optical system, cost, and size. In addition, in multi-port reception, the characteristics of the respective reception ports, such as transmission delay time, coupling efficiency, and gain must be matched with each other.
In contrast to this, in (4) polarization scrambling (T. G. Hodgkinson, R. A. Harmon, and D. W. Smith, "Polarization Insensitive Heterodyne Detection Using Polarization Scrambling", Electronics Letters, Vol. 23, No. 10, pp. 513-514, May 1987) or polarization switching (I. M. I. Habbab and L. J. Cimini, Jr., "Polarization-Switching Techniques for Coherent Optical Communications", Journal of Lightwave Technology, Vol. 6, No. 10, pp. 1537-1548, October 1988), the polarization state of signal light or local oscillation light is changed in a time slot of one bit, and an average value is obtained. This method has a receiving sensitivity lower than that of other methods by 3 dB and cannot be applied to a system of a high data rate, but the receivers are much simple. Therefore the method is expected to be applied to a subscriber's system and the like with a relatively low data rate. The following polarization scrambling (switching) light sources have been reported:
The polarization bistable semiconductor laser (a), however, cannot be suitably applied to coherent optical communication because the wavelength of output light or the output power generally varies during a polarization switching operation. The external modulator (b) is not suitable for practical applications because it has various drawbacks, e.g., a large insertion loss, a large driving voltage, difficulty in high-speed switching, poor temperature characteristics, poor stability, and low reliability. In addition, the polarization switch based on laser frequency modulation (c) causes an increase in spectrum line width, resulting in difficulty in coherent detection. Since the problems with (a) and (b) seem to be apparent, only the problems with (c) will be described below with reference to the following examples.
In a polarization switching heterodyne receiver using a frequency switch of a local oscillation laser, an oscillation frequency f.sub.L of the local oscillation laser is frequency-modulated by a rectangular wave to become f.sub.L1 in the first half of a one-bit time slot and f.sub.L2 in the second half. A local oscillation laser output passes through a polarization maintaining optical fiber of birefringence B=n.sub.x -n.sub.y and is mixed with signal light having a frequency f.sub.S by an optical fiber coupler. A beat signal (IF signal) based on signal light and local oscillation light generated by a photodiode is demodulated into a baseband signal by an IF circuit and a demodulation circuit. In this case, the main axis (x axis) of the polarization maintaining optical fiber is inclined at 45.degree. with respect to the polarization direction of the input local oscillation light. A length L of the polarization maintaining optical fiber is set to be L=c.sub.0 /(2B.DELTA.f) (where c.sub.0 is the velocity of light in a vacuum and .DELTA.f=f.sub.L1 -f.sub.L2. If, for example, B=5.times.10.sup.-4 and .DELTA.f=1 GHz, then L=300 m.
A phase retardation .DELTA..theta.(f) between the x- and y-axis polarization components of output light from the polarization maintaining optical fiber is 2.pi.LBf/c.sub.0. The difference in the phase retardation for two frequencies f.sub.1 and f.sub.2 is given by EQU .DELTA..theta.(f.sub.1)-.DELTA..theta.(f.sub.2)=.pi. (1)
The polarization states (P.sub.1, P.sub.2) of light having a .pi. difference in .DELTA..theta. are orthogonal. That is, a polarization switch for output light is realized.
In this method, however, a frequency change occurs together with polarization switching of output light. When differential phase shift keying (DPSK) or amplitude shift keying (ASK) is to be performed, the IF frequency can be fixed to f.sub.IF =(f.sub.1 -f.sub.2)/2 by setting f.sub.S =(f.sub.1 +f.sub.2). In frequency shift keying (FSK), however, two IF frequencies appear in accordance with polarization states. In the above case, since two IF frequencies having a difference .DELTA.f=1 GHz appear, a wide IF band must be set to cover the two frequencies. For this reason, a frequency deviation of 2 GH or more must be set. In addition, an automatic frequency control (AFC) circuit for stabilizing the IF frequency is inevitably complicated.
The x- and y-axis polarization components of light output from the polarization maintaining optical fiber has a propagating time difference represented by .tau.=LB/c.sub.0 =1/(2.DELTA.f). When polarization switching is to be performed by this method, in order to sufficiently reduce the time during which light having a frequency f.sub.1 and light having a frequency f.sub.2 overlap each other, the transmission time difference .tau. must be set to be .tau.&lt;&lt;T/2, i.e., the bit rate must be set as R.sub.B =1/T&lt;&lt;.DELTA./2. In this case, R.sub.B &lt;&lt;1 Gb/s. This means that a modulation index m must be set to be large in FSK. For example, at 100 Mb/s, modulation with a frequency deviation of 2 GHz (m=20) is required. The method described above requires a wide IF band and causes a great deterioration in sensitivity and hence is not suitable for practical applications.
Furthermore, since the output power of a semiconductor laser generally varies with frequency modulation, a beat signal is unbalanced between two polarization states, and the output exhibits slight polarization dependency. Although a laser structure and a modulation method may be designed such that frequency modulation can be performed with a constant output, a driving circuit and an output feedback circuit become complicated.
(b) Problems associated with phase noise and receiver band width.
The drawback of the heterodyne detection scheme is that a receiver having a wide band is required because an IF band is used. Especially in transmission at a high bit rate, the band width of a receiver is a bottleneck. In addition, in consideration of the application of the scheme to a subscriber's system, even at a low bit rate, requirement of a wide band for a receiver is not preferable in terms of cost.
In contrast to this, in the homodyne detection scheme which requires no IF band, no demerits are found in terms of a receiver band width. However, the scheme is greatly influenced by phase noise from a light source. In an extreme case, if signal light and local oscillation light have a phase difference of 90.degree., the reception output is zero. Although the spectrum line width of a semiconductor laser is beginning to be decreased, it is still difficult to realize a full width half maximum of 100 kHz or less without using, e.g., a special feedback loop or an external resonator. Such a feedback loop or an external resonator leads to an increase in the size of a light source, an increase in the number of portions to be adjusted, a deterioration in stability and reliability, and an increase in cost. Even if the line width is reduced by such a means, the phase noise cannot be reduced to zero. For this reason, an optical phase locked loop (optical PLL) is required to match the phase of local oscillation light with that of signal light. Currently, however, such a means is very difficult to realize.
A great deal of attention, therefore, is paid to a phase diversity reception scheme (e.g., L. G. Kazovsky, "Phase- and Polarization-Diversity Coherent Optical Techniques", Journal of Lightwave Technology, Vol. 7, No. 2, pp. 279-292, February 1989) as a scheme capable of receiving signal light in the baseband without using an optical PLL. In this method, components of a plurality of phases of signal light are independently received, and the received components are electrically synthesized in the same manner as in polarization diversity. Similar to a polarization diversity receiver, however, a complicated arrangement is required, and the characteristics of the respective ports must be matched with each other. In the phase diversity reception scheme, the problem of polarization matching is also posed. The arrangement for polarization diversity and phase diversity requires at least four optical receivers.
In order to prevent this problem, phase switching reception may be employed (I. M. I. Habbab, J. M. Kahn, and L. J. Greenstein, "Phase-Insensitive Zero-IF Coherent Optical System Using Phase Switching", Electronics Letters, Vol. 24, No. 15, pp. 974-976, June 1988). Even the phase switching scheme is not free from the problem of polarization matching. In addition, no method of simultaneously performing phase switching and polarization switching has yet been established.
(c) Problems associated with high density optical FDM
The number of channels which can be multiplexed by an optical FDM is determined by a usable frequency (wavelength) band and an allowable channel interval. The frequency band is limited by the tuning range of an local oscillation source or the band of an optical system. Although the tuning range has been increased with the advances in variable-wavelength semiconductor lasers, it is not sufficiently wide yet. A reduction in channel interval is indispensable for the effective use of a wavelength band. Especially in heterodyne reception, the presence of an image component in an IF band limits a channel interval. A reception scheme (image removing receiver) for removing this image component has been proposed. However, if this scheme is combined with polarization diversity, the resulting arrangement is very complicated. Such an image removing receiver is disclosed in Terumi Chikama et al., "Optical Heterodyne Image Rejection Receiver", 1989 Spring National Convention Record, the Institute of Electronics, Information and Communication Engineers of Japan, Part 4, pp. 4-133, 1989. According to this scheme, an optical system has a very complicated arrangement, and an electrical system also has a complicated arrangement with four balanced light receivers. The application of such a scheme to a multichannel optical CATV distribution system or an optical LAN is very difficult in terms of, e.g., reliability and cost.
The problems in the practical application of the coherent optical communication scheme are associated with: (a) polarization matching, (b) phase noise and a receiver band width, and (c) a high density FDM. As methods of solving these problems, the methods based on the following means are regarded to be effective: (A) polarization diversity, (B) phase diversity, and (C) multi-port reception using an image removing receiver and the like. However, in the method based on multiport reception, a complicated arrangement is required, and the characteristics of the respective ports must be matched with each other, thus undesirably restricting allowable characteristic specifications. If two or more of the methods (A), (B), and (C) are to be simultaneously performed, a very complicated arrangement is required. The complication of an optical system causes an increase in the number of portions to be adjusted, a deterioration in reliability, and a great increase in cost and hence poses a serious problem in terms of practical applications. Even in an electronic circuit system, the complication of the arrangement of a high-frequency circuit poses a serious problem, as in hetero-dyne reception.
As other methods of solving the problems (a) and (b), (D) a polarization switching (scrambling) method and (E) a phase switching method have been proposed. In the conventional switching methods, however, a high-speed, low-power-consumption operation is difficult to realize, and the spectrum line width is undesirably increased. Therefore, many limitations are imposed on the application of the methods. For example, it is difficult to apply the methods to FSK. That is the practical value of the methods is low. In addition, the methods exhibit poor compatibility with image removing signal reception.